The situation has improved.
We were on 3.6.0, Postgres 13 was working without evident issues - so far.
We posticipated the upgrade to 15 because we didn’t have enough free space on the vps, we needed 150GB given our database size.
We pruned what we considered useless data - tracking data and search data among all -, performed some routine maintenance on the DB (VACUUM, for instance), upgraded to PSQL15, and upgraded Discourse too, to 2026.2.
We don’t know what exactly fixed the issue but we’re doing fine now.
The situation seems to be stable now.
I’m going to monitor usage metrics for a couple days more.
Thanks for your help so far.